1. Technical Field
This invention relates to an optical detection apparatus and method of detecting electromagnetic fields. More particularly, this invention relates to a magneto-optical detection apparatus and method of detecting weak magnetic fields that offers a high degree of spatial resolution, and operates at room temperature.
2. Background Information
The ability to detect weak magnetic fields provides access to several important medical and industrial applications. Perhaps foremost among these is functional imaging in humans. Virtually all of the biomedical imaging modalities clinically available at present (MRI, CT, PET, ultrasound, etc.) are invasive (active) in the sense that each requires the patient to be irradiated, bathed in an intense (>1 T) magnetic field, or to ingest a radioactive substance. Furthermore, most of these modalities are quite expensive, requiring facilities such as magnets, large detector arrays, access to short-lived radioisotopes, or X-ray generators (which rely on high voltages) that are essentially immobile and within the financial means of only the larger medical centers and clinics. Finally, several prevalent modalities do not readily provide functional information.
Detecting the magnetic fields produced by various organs and systems is one of the few opportunities the human body offers to passively monitor its functions. In 1962, Baule and McFee demonstrated that the magnetic field produced by the human heart could be measured outside the torso with two large (˜1 ft. in length) coils. They coined the term magnetocardiograph and suggested that imaging the heart's magnetic field might provide functional information. Eight years later, Cohen abandoned the use of coils for the detector in favor of the superconducting quantum interference device (SQUID) and observed the magnetic fields produced by the heart and brain (alpha rhythm; peak ˜25 nG) with a detection floor of ˜1 nG. Since Cohen's pioneering work, SQUID-based magnetocardiography (MCG) and magnetoencephalography (MEG) systems have been developed steadily and, as of July 2000, six SQUID systems having as many as 74 devices each had been constructed and were devoted to MCG research. Systems with much larger numbers (≦256) of devices are currently available and have proven to be valuable clinical tools for the diagnosis and study of a variety of neurological and cardiovascular conditions. Furthermore, biomagnetic susceptometry with SQUID magnetic detectors is a clinically-proven technique for measuring iron stores in the human body. However, MCG and MEG systems suffer from several obvious drawbacks. Foremost among these is the Josephson junction at the heart of the SQUID which requires cooling to liquid He (i.e., cryogenic) temperatures. Furthermore, the spatial resolution of the SQUID is limited by the device itself and its associated sensing loop to typically 1 cm2, leading to a minimum 4 cm spacing between devices. Consequently, although MCG and MEG have proven to be successful in imaging the human heart and brain electrical activity (respectively), and measuring human tissue iron stores, SQUID technology is bulky and prohibitively expensive for small hospitals and rural clinics. Recently, sensitive magnetometers based on laser-excited metal vapors (such as rubidium) have been developed, but to date have not been applied clinically.
In summary, the only clinical tool that has been developed for MCG and MEG is the SQUID-based system. Despite its extraordinary sensitivity (detection floor of ˜10−15 T or ˜10 pG), the barriers to widespread (e.g., low cost) use of this technology are formidable. Nevertheless, if an inexpensive technique for detecting and imaging the weak magnetic fields produced in humans became available, it would be a valuable clinical asset since the imaging process is non-invasive and inherently provides functional information. Not only would such a system be of value for low cost imaging of the magnetic fields produced naturally by various organs and systems of the human body but it would also be useful in tracking magnetic particles intentionally introduced to the human circulatory or intestinal systems, for example, for a variety of clinical purposes.